It's more likely that there's some dependency that is not easy to upgrade, because the upgrade also includes api changes (from the dependency, not from the jdk).

Compound this with multiple dependencies that exhibit this issue. If you have a "legacy" application that does not require active development, there's zero business incentive to invest into the upgrade. Unless you could prove value in having the upgrade, it just doesn't get prioritized.

Java has in recent times actually removed some apis from the standard library, and made some inaccessible (undocumented internal apis especially) so any advanced frameworks that use them would have been affected.